Examples of progress made during the prior year are summarized below. 1. Replication of the finding that PET radioligand binding to TSPO, a putative biomarker of inflammation, is increased during major depressive episodes Translocator protein 18 kDa (TSPO), a potential biomarker of neuroinflammation, can be accurately quantified using positron emission tomography (PET) and 11CPBR28, a TSPO tracer. A recent study found increased TSPO binding in multiple brain regions in unmedicated subjects with major depressive disorder (MDD) currently experiencing a major depressive episode (MDE). Building on this work, we sought to: 1) replicate these findings using a different patient population and a different tracer; 2) investigate the effects of antidepressants on TSPO binding in these subjects; and 3) determine the relationship of peripheral and central inflammatory markers to TSPO binding. Towards this end, unmedicated MDD subjects (n = 12), medicated MDD subjects (n = 16), and healthy controls (n = 20) underwent PET imaging using 11CPBR28. Total distribution volume (VT) was measured using arterial input function and corrected for TSPO genotype. The subgenual prefrontal cortex and anterior cingulate cortex were chosen as regions of interest. Peripheral blood samples and cerebrospinal fluid were analyzed to investigate the relationship between peripheral and central inflammatory markers and TSPO binding. We found that TSPO binding was higher in MDD subjects versus healthy controls in both the subgenual prefrontal cortex as well as the anterior cingulate cortex, though these comparisons missed the corrected threshold for statistical significance (=.025). Exploratory analyses demonstrated that unmedicated MDD subjects had the highest levels of TSPO binding, followed by medicated MDD subjects, who did not differ from healthy controls. TSPO binding correlated with interleukin-5 (IL-5) in cerebrospinal fluid but with no other peripheral or central inflammatory markers. The findings reveal an important trend towards increased TSPO binding in the brains of MDD subjects, and the post hoc analysis extended these findings by demonstrating that this abnormality was significant in unmedicated (but not medicated) MDD subjects. Taken together, our results replicated previous findings showing increased TSPO binding in the brain of MDD subjects experiencing an MDE, suggesting that inflammation is a key target for developing novel therapeutics to treat MDD. The findings are particularly important because they may help further elucidate pathways involved in the development of MDD as well as identify potential novel treatments and pharmacological targets. This work was performed under clinical protocol NCT01851356. 2. 11C-ER176, a newly developed radioligand for 18-kDa translocator protein (TSPO), has adequate sensitivity to successfully image all three affinity genotypes in human brain Previous work from our laboratory demonstrated that most second-generation positron emission tomography (PET) radioligands used to measure 18-kDa translocator protein (TSPO), a biomarker of neuroinflammation, were sensitive to the single nucleotide polymorphism (SNP) rs6971. To address this issue, our laboratory developed 11C-ER176; we found that this ligand showed little sensitivity to rs6971 when tested in vitro and, furthermore, had high specific binding in monkey brain. We then sought to determine whether the sensitivity of 11C-ER176 in humans was similar to the low sensitivity measured in vitro. A secondary goal of the study was to measure the nondisplaceable binding potential (BPND, or the ratio of specific-to-nondisplaceable uptake) of 11C-ER176 in human brain. Towards this end, nine healthy volunteers, three high-affinity binders (HABs), three mixed-affinity binders (MABs), and three low-affinity binders (LABs) were studied with whole-body 11C-ER176 PET imaging. Standardized uptake values (SUVs) from 60 to 120 minutes after injection derived from each organ were compared between genotypes. In addition, eight separate healthy volunteers, three HABs, three MABs, and two LABs underwent brain PET imaging. The three HABs underwent a repeated brain scan after TSPO blockade with XBD173 to determine nondisplaceable distribution volume (VND) via Lassen occupancy plotting and thereby estimate BPND in brain. We found that regional SUVs averaged from 60 to 120 minutes post- injection in brain and peripheral organs with high TSPO densities (for instance, lung and spleen) were greater in HABs than in LABs. On the basis of VND determined via the occupancy plot, the whole-brain BPND for LABs was estimated to be 1.4 0.8, which was much lower than that for HABs (4.2 1.3) but about the same as that for HABs with 11C-PBR28 (1.2). These important findings suggest that a clear in vivo sensitivity to rs6971 was observed with 11C-ER176 that had not been expected from in vitro studies. Notably, this suggests that the future development of any improved radioligand for TSPO should consider the possibility that in vitro properties will not be reflected in vivo. We also found that 11C-ER176 had adequately high BPND for all rs6971 genotypes. Thus, the new radioligand would likely have greater sensitivity for detecting abnormalities in patients. This work was performed under clinical protocols NCT02147392 and NCT02181582. 3. Decreased cannabinoid CB1 receptors in male tobacco smokers examined The Molecular Imaging Branch (MIB) previously conducted several studies that identified reductions of brain cannabinoid CB1 receptors in adults with cannabis and alcohol use disorders. In addition, preclinical studies suggested that these cannabinoid receptors also contributed to nicotine reward and dependence. Of particular relevance to the MIBs work with neuropsychiatric populations, many patients with such disorders smoke tobacco, which may confound clinical studies of psychiatric disorders. We conducted a study to ascertain whether human subjects who smoke tobacco but are otherwise healthy have altered CB1 receptor binding in brain. Towards this end, we measured CB1 receptors in the brains of 18 healthy men who smoke tobacco (frequent chronic cigarette smokers), and 28 healthy men who do not smoke tobacco, using positron emission tomography (PET) and 18FFMPEP-d2, a radioligand for CB1 receptors. We collected arterial blood samples during scanning to calculate the distribution volume (VT), which is nearly proportional to CB1 receptor density. Repeated measures analysis of variance compared VT between groups in various brain regions. Our results indicated that brain CB1 receptor VT was about 20% lower in subjects who smoked tobacco than in subjects who did not. Decreased VT was found in all brain regions, but reductions did not correlate with years of smoking, number of cigarettes smoked per day, or measures of nicotine dependence. The results indicate that tobacco-smoking healthy men had a widespread reduction in CB1 receptor density in brain. Because CB1 receptor reductions appear to be a common feature of substance use disorders, the findings suggest that future clinical studies of CB1 receptors should control for tobacco smoking. This work was performed under clinical protocol NCT00816439.